Deep L′- and M-band imaging for planets around Vega and ε Eridani

We have obtained deep adaptive optics (AO) images of Vega and ε Eri to search for planetary mass companions. We observed at the MMT in the L′ (3.8 μm) and M (4.8 μm) bands using Clio, a recently commissioned imager optimized for these wavelengths. Observing at these long wavelengths represents a departure from the H band (1.65 μm) more commonly used for AO imaging searches for extrasolar planets. The long wavelengths offer better predicted planet/star flux ratios and cleaner (higher Strehl) AO images at the cost of lower diffraction-limited resolution and higher sky background. We have not detected any planets or planet candidates around Vega or ε Eri. We report the sensitivities obtained around both stars, which correspond to upper limits on any planetary companions which may exist. The sensitivities of our L′- and M-band observations are comparable to those of the best H-regime observations of these stars. For ε Eri, our M-band observations deliver considerably better sensitivity to close-in planets than any previously published results, and we show that the M band is by far the best wavelength choice for attempts at ground-based AO imaging of the known planet ε Eri b. The Clio camera itself, with MMTAO, may be capable of detecting ε Eri b at its 2010 apastron, given a multinight observing campaign. Clio appears to be the only currently existing AO imager that has a realistic possibility of detecting ε Eri b.